1. Technical Field of the Invention
This invention relates generally to submersible well pumping systems. This invention relates particularly to a positive displacement pumping system with an improved, efficient and reliable flow switching mechanism.
2. Description of the Background Art
Hydraulically driven diaphragm pumps are positive displacement pumps that are nearly immune to the effects of sand in the pumped fluid because the pressure generating elements are isolated from the pumped fluid by a flexible diaphragm. In well pump applications, this type of pump is driven by a self contained, closed hydraulic system, activated by an electric or hydraulic motor where the pump, closed hydraulic system, and the motor are enclosed in a common housing and submerged in a well. There are many examples of this type of well pump in the patent literature, but currently none are in use as well pumps because of high cost and/or poor reliability. In well pump applications, the key design feature is the switching mechanism used to redirect or reverse the flow of working fluid from the fluid source, referred to as the auxiliary pump, to the working fluid sub-chambers. The reversal of the flow causes the pumped fluid to  move into and out of-pumped fluid sub-chambers through check valves, accomplishing the pumping action.
U.S. Pat. No. 2,435,179 discloses a hydraulically driven diaphragm pump which uses a hydraulically actuated valve to reverse the flow of working fluid. The valve is driven by differential pressure between the fluid inside the working diaphragm (working fluid) and the fluid outside the working diaphragm (pumped fluid). Normally, no differential pressure exists between the two volumes. The pump creates the differential pressure required to reverse the pump by completely filling the diaphragm, causing it to stretch after it is completely full. The amount of pressure generated is limited by the strength of diaphragm material and has the disadvantage of creating diaphragm stress, which can lead to premature diaphragm failure. To maximize diaphragm life, this differential pressure must be limited to the lowest level possible.
The '179 patent uses two sets of diaphragms, one set to control the valve, and the other set to achieve pumping. The pumping diaphragms are located inside the pumping chambers, and the control diaphragms are located between the working fluid inside the pumping chambers and the pump outlet. The external surfaces of the working and control diaphragms are separated by an outlet check valve, creating the possibility that the external pressure would be higher on the control diaphragm due the presence of the checkvalve. If the inlet pressure is higher than the outlet pressure (a common occurrence in well pumps) the pump will not operate and could be damaged. This situation commonly occurs when the pump is installed in a standing fluid column, before fluid has a chance to equalize by flowing through the pump checkvalves. This arrangement also limits the usefulness of the pump by limiting it to use in conjunction with a large diameter liner rather then a more conventional, smaller diameter drop pipe.
A more significant problem occurs in low volume applications. The nature of the pump requires that the hydraulically actuated switch be driven by the same pressure source controlled  by the valve, which causes the valve driving force to be released when the valve transverses an intermediate position between states. In low volume applications, the valve can stop in an intermediate position before it has completely reversed the pump. This can cause the pump to either dither (rapid but incomplete movement of the working fluid in one direction), or go into a mode where half the flow is directed into each chamber or stops, which causes the pump to stop functioning. This is especially true during low volume, near equilibrium operations such as those that occur when stopping and starting the pump.
Other problems will occur with the valve setup disclosed in the '179 patent. For example, the control diaphragm is acting directly on a tappet, leading to fluid accumulation between the diaphragm and the tappet, which in turn leads to diaphragm failure unless measures are taken to relieve the fluid. For these and other reasons, the pump described in the 179 patent has never been used in a practical application. This patent application addresses those shortcomings and describes a practical well pump with in improved sensor.
U.S. Pat. No. 2,961,966 discloses another method to reverse the flow of working fluid by reversing the direction of rotation of the electric motor driving the auxiliary pump. That patent discloses a method to sense the differential pressure between the working fluid and the pumped fluid to activate the electrical braking and reversal of the electric motor driving the auxiliary pump. That method also leads to diaphragm stress because differential pressure is required across the diaphragm to actuate the sensor. In addition motor reversal requires very complex electronics. Although theoretically possible, in practice the complexity of that method leads to high expense and unreliable operation due to the difficulty of controlling and reversing the electric motor in a downhole environment. 
U.S. Pat. No. 6,017,198 discloses another method to reverse the flow of working fluid, namely the use of sensors and electronics to detect the fact that the diaphragm is full, and reverse the direction of flow by using an electrically actuated valve. This method works very well, but requires relatively complex electronics and a connection into the main power cable. Sealing electronics and power cables against high ambient pressure environments found in wells is expensive and can lead to premature failures of the pump due to high ambient pressure related electrical shorts.
What is needed is a switch that is simple yet reliable, and will not stick in an intermediate position, especially during low volume or start-stop cycles. The most effective switch is a passive hydraulic switch; that is one that derives the power to operate the switch from the flow of hydraulic fluid through the switch. These type switches have proven to be unreliable during low flow and start-stop operations when of conventional design. This application discloses and claims a submersible well pumping apparatus equipped with a reliable flow switch that is either passive hydraulic or contains a minimum of electrical elements.